Police in the southern U.S. state of Tennessee said a shooter who killed three children and three adults at a Christian grade school Monday had carried out surveillance before the attack.

Metropolitan Nashville Police Chief John Drake told reporters Monday evening that the shooter, who was shot dead by police, had drawn a detailed map of the school, including potential entry points.

He identified the shooter as 28-year-old Audrey Hale, a former student at the school. Hale left behind a 'manifesto' and other writings, which investigators are examining, Drake said.

Don Aaron, a spokesperson for the police department, said the shooter was armed with at least two semi-automatic rifles and a handgun.

He said police began receiving calls at 10:13 a.m. about a shooter at The Covenant School, a private Presbyterian school in Nashville.

A child weeps while on the bus leaving The Covenant School, following a mass shooting at the school in Nashville, Tennessee, March 27, 2023. (Nicole Hester/USA Today Network via Reuters)

Officers who arrived at the scene could hear gunfire coming from the school's second floor, according to Aaron.

He said two officers who were part of a five-member team shot the shooter in a lobby area and that Hale was dead by 10:27 a.m.

Police described the victims as three students - all 8 or 9 years old - and three adult staff members. They said besides those six people, no one else was shot.

Police escorted the other students to a nearby church, where parents were told to gather.

Children from The Covenant School hold hands as they are taken to a reunification site at the Woodmont Baptist Church after a deadly shooting at their school, in Nashville, Tennessee, March 27, 2023.

The Covenant School has about 200 students from pre-school to sixth grade, according to its website.

'It's heartbreaking, a family's worst nightmare,' President Joe Biden told reporters in Washington.

He again urged Congress to pass more gun control legislation.

'We have to do more to protect our schools, so they aren't turned into prisons ... I call on Congress again to pass my assault weapons ban,' he said.

First lady Jill Biden also spoke about the shooting Monday.

'I am truly without words. And our children deserve better,' she said during a National League of Cities conference in Washington.

Nashville Mayor John Cooper tweeted, 'In a tragic morning, Nashville joined the dreaded, long list of communities to experience a school shooting. My heart goes out to the families of the victims. Our entire city stands with you.'

VOA SPECIAL REPORT: History of Mass Shooters

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Last year, a mass shooting at an elementary school in Uvalde, Texas, left 21 people dead.

Other deadly school massacres in the United States include a 2018 high school shooting in Parkland, Florida, in which 17 people died; a 2012 assault at Sandy Hook Elementary in Connecticut in which 26 people were killed; and a 1999 shooting at Columbine High School in Colorado that left 13 dead.

Some information in this report came from The Associated Press and Reuters.